In the paper-making process, the process itself has long deadtimes relative to the process time constant. This makes control difficult. There are typically two unique deadtimes; one is for the time required for a change in basis weight when the input variable, stock flow, is changed and the other is the time for variation in steam to affect the final moisture carried by the paper sheet. A further difficulty in the paper-making process is the cross-coupling affect; that is, each input variable affects both output variables. Hence, a decoupling controller is desired to regulate the outputs independently; for example, the operator would like to change the setpoint of the basis weight controller without changing the value of the moisture.
Two rather crude techniques have been utilized for decoupling. In a first, a setpoint is changed only once every five minutes, for example, for changes in stock flow and once every minute for changes in steam. This is a much longer period of time than the generation of output data by a sensor which scans across the width of the paper, for example, every 20 seconds.
A second proposed decoupling technique is to provide absolute decoupling constants between changes in stock flow and steam pressure. These might be termed "compensating changes". However, these are mere guesses and do not compensate for grade changes or speed changes and do not take into account that the coupling effect may be nonlinear. One other problem with a controller for a paper-making machine discussed above, is the fact that the measurements of outputs occur either at long intervals or can occur asynchronously due to sheet breaks or standardization. In any case, scan measurements (which may take up to 120 seconds) only occur every 20 seconds at best.